Most of electric and electronic appliances generate heat while they are used, so that the removal of the generated heat therefrom is necessary for their normal operation. With the intention of removing the generated heat from those appliances, many means have been proposed. For instance, in miniature electronic parts, especially electronic devices provided with integrated circuits, thermally conductive materials, such as a thermally conductive grease and a thermally conductive sheet [Japanese Tokko Sho 57-36302 (U.S. Pat. No. 4,265,775), wherein the term "Tokko" means an "examined patent publication", and Japanese Tokkai Sho 61-157587, wherein the term "Tokkai" means an "unexamined published patent application"], have been used as such means.
In general, an electronic device comprises integrated circuits and cap parts for protecting them, and a thermally conductive material is applied so as to contact directly with both the circuit element and the heat reducing part, or indirectly therewith via certain materials. Thus, the heat generated from integrated circuit chips during operation is transmitted in the thermally conductive material to be transferred directly or indirectly to the heat reducing part, and further radiated therefrom. The rough sectional view of such an electronic device, wherein a thermally conductive material is used, is shown in FIG. 1.
As the aforementioned thermally conductive material, there has already been known a heat-reducing grease of the type which uses a silicone oil as a base material and a zinc oxide or alumina powder as a thickener (Japanese Tokko Sho 52-33272 and Japanese Tokko Sho 59-52195). In recent years, aluminum nitride has been developed as a thickener which enables further improvement of thermal conductivity (as disclosed, e.g., in Japanese Tokkai Sho 52-125506).
The foregoing silicone compositions in a state of grease are prepared by mixing a silicone oil with a thickener having a low affinity for the silicone oil, so that they have a problem of separating the oil from the composition (in terms of the degree of oil separation) upon long-term standing at a high temperature or by a long-range repetition of cooling and heating cycles, and so on. This problem frequently arises in cases where the thickener used has a relatively large particle size and excellent thermal conductivity.
Therefore, various studies have so far been given to the problem of discovering a thermally conductive grease-state silicone composition having reduced oil separation. As a result, the thermally conductive materials as described below have been disclosed; for instance, one material disclosed is the silicone grease composition which comprises 10-50 parts by weight of an organopolysiloxane modified by 2-phenylethyl, 2-phenylpropyl or 6-30C alkyl groups and 90-50 parts by weight of a metal oxide, such as silica, diatomaceous earth, zinc oxide, alumina or titanium oxide (Japanese Tokkai Sho 51-55870), and another material disclosed is the thixotropic thermally conductive material which comprises an oily organosilicone carrier, a thermal conductivity-providing filler powder selected from a group consisting of thin-leaf aluminum nitride, dendrite-form zinc oxide, thin-leaf boron nitride and a mixture of two or more thereof and a silica fiber acting as an exudation inhibitor [see Japanese Tokko Sho 57-36302 (U.S. Pat. No. 4,265,775)].
Other materials disclosed are the thermally conductive silicone grease composition comprising an organopolysiloxane, silicon carbide and aerosol silica (see Japanese Tokkai Sho 62-43492), and the thermally conductive silicone oil compound comprising a hydroxyl group-containing organopolysiloxane having a viscosity of 10 to 100,000 cs wherein the hydroxyl groups comprise 5-50 mole % of the total end groups and a powder of at least one metal compound selected from a group consisting of zinc white, alumina, aluminum nitride and silicon nitride [see Japanese Tokkai Hei 2-212556 (U.S. Pat. No. 5,221,339)].
Still another material disclosed is the heat-reducing silicone grease composition which comprises an organopoly-siloxane having a viscosity of 5 to 500,000 cs, a MQ resin constituted of R.sub.3 SiO.sub.1/2 units, R.sub.2 SiO units and SiO.sub.2 units and having the R.sub.3 SiO.sub.1/2 /SiO.sub.2 ratio in the range of 0.5:1 to 2:1 by mole and a thickener selected from the group consisting of zinc oxide, alumina, aluminum nitride, boron nitride and silicon carbide [Japanese Tokkai Hei 3-162493 (U.S. Pat. No. 5,100,568)].
However, the grease-state silicone compositions disclosed in Japanese Tokko Sho 57-36302 (U.S. Pat. No. 4,265,775) and Japanese Tokkai Sho 62-43492 have a drawback of being insufficient in thermal conductivity because they contain as an additive a silica fiber and aerosol silica respectively, and thereby the content of the thickener having high thermal conductivity is reduced. And the grease-state silicone compositions disclosed in Japanese Tokkai Sho 51-55870 and Japanese Tokkai Hei 2-212556 (U.S. Pat. No. 5,221,339), though they use particular organopolysiloxanes, are still unsuccessful in satisfactory reduction of oil separation. On the other hand, the heat-reducing silicone grease composition disclosed in Japanese Tokkai Hei 3-162493 is successful in prevention of oil exudation, but has a problem that the thermal conductivity cannot be raised up to a satisfactory extent because the composition is rather hard due to the tackiness of MQ resin contained therein to make it difficult to increase the thickener content due to viscosity limit placed on the composition as grease.